Coal is an important fossil fuel used, for instance, in the large scale generation of electricity at coal-fired power stations. Consisting mainly of carbon, coal releases carbon dioxide when it is combusted; a recognised greenhouse gas. Coal also contains other extraneous materials, therefore its combustion produces other gases and ash as by-products and some of these may pollute the environment.
The end use of coal can be determined by reference to its Maceral composition. Macerals are the metamorphised remains of the vegetation matter from which the coal was originally formed. There are three main groups of Macerals recognised in coals: Vitrinite, Liptinite (previously named Exinite) and Inertinite. These are in turn subdivided according to the detailed character of the Maceral's material.
The major types of coal in the world, Humic and Sapropelic, covering Cannel and Boghead coals, are defined and identified by the number and combination of the three Macerals that they contain.
Each of the three Maceral groups have different characteristics and properties such as Density, Specific Energy, Volatile Matter, Fixed Carbon and Ash Contents. Macerals generally fall into two classes based on their contribution to the combustion of coal—the Reactive and Inert classes. The Reactive Macerals, Vitrinite and Liptinite, display high Specific Energies and very low ash contents. Inertinite also actively participates in the combustion of coal, but to a lesser degree because it has a higher ash content.